docs/develop/packages/TODO: add missing part

Sorry, apparently I was half-asleep and missed this part

Change-Id: I888a975ae7ff30d1039f466e63d37c30b94d3739
Reviewed-on: https://review.haiku-os.org/444
Reviewed-by: waddlesplash <waddlesplash@gmail.com>

docs/develop/packages: convert wiki to rst

Cleaner syntax compared to MediaWiki, more features than Markdown, while
still looks good on plain text.